The present invention relates generally to cardiac pacing, and more particularly to an implantable cardiac pacemaker which is implemented with the capability to provide bidirectional communication not only with a programmer or telemetry receiver external to the patient's body but with the patient himself.
Cardiac pacemakers generally provide means for sensing electrical signals generated by the heart to control the stimulation of excitable tissue in the chambers, and for sensing the cardiac response to such stimulation (e.g., intracardiac signals), and means for responding to an inadequate or inappropriate stimulus or response (e.g., dysrhythmia) to undertake delivery of therapeutic stimuli to the patient's heart. This generalized function exists regardless of whether the implanted device is intended for pacing alone or has additional capabilities of cardioversion or defibrillation, to alleviate other dysrhythmias such as pathologic tachycardia or fibrillation.
A current matter of concern in cardiac pacemakers resides in the safety of pacer interaction with the patient. Despite improved cardiac pacemaker technology, the pacemaker patient may still suffer from symptoms of an underlying cardiac disease after the pacemaker has been implanted. For example, the patient may experience a symptomatic, unpleasant, irregular heartbeat. The attending physician must evaluate the symptoms and diagnose their underlying cause. A reliable evaluation, however, may be frustrated by the fact that the symptoms occur only infrequently. That is to say, the patient may be event- and symptom-free for a substantial period of time, e.g., several months; but at some point in time the previously suffered unpleasant symptoms may recur. 24-hour monitoring of the patient is not a viable solution in these circumstances because the likelihood of detecting such a sporadic rhythm disorder is extremely low. But a technique of recording the patient's intracardiac signal from time to time or which is triggered upon the occurrence of a certain rhythm disorder could be fruitful.
Reliable detection of the intracardiac signal requires that the electrode(s) must have and maintain proper electrical contact with the tissue of the heart, and that the device leads are operating properly. Such detection will be frustrated or may fail entirely in the event of failure of a lead, such as an incomplete lead fracture which results in only intermittent loss of signal, or a defect in the lead insulation, or improper placement of the electrode that results in unreliable contact with myocardial tissue in the chamber of interest. In such cases, it would be helpful to have available a real-time electrocardiogram (ECG) signal or chart that is not restricted to endocardial information derived from the pacing (or other function) leads. Of course, a surface electrocardiogram would provide alternative or additional verification of the status of the patient's cardiac activity and the operation of the implanted pacemaker, but such an alternative is not particularly viable where the patient is suffering from symptoms attributable to intermittent failures of the implanted pacemaker.
It is a principal aim of the present invention to provide an implantable cardiac pacemaker or other implantable cardiac therapeutic medical device which is capable of detecting or permitting the detection of faults in the implanted lead system associated with the device or the device itself, and which is implemented to do so in a simple and low cost manner. An additional objective is to provide means for alerting the patient to a problem in the implanted system and/or to more readily enable the patient to take manual action to temporarily override certain operations of the implanted device in response to cardiac dysrhythmias which are not life threatening.
The implanted pacemaker may be implemented to perform other functions as well, such as cardioversion and defibrillation. Such multi-function implantable medical devices are well known. In the instance of a device which is capable of detecting atrial fibrillation, such fibrillation will be detected by the device but may not be sensed by the patient himself. Indeed, this is a frequent occurrence. Generally, implantable devices which detect atrial fibrillation are implemented to deliver a defibrillating shock to the atria, which can be painful to the patient. Atrial fibrillation is not an immediate life-threatening disorder, but if left untreated for 24 to 48 hours the patient will require anticoagulation (i.e., the administering of an anticoagulant agent such as heparin or hirudin into the bloodstream) to avert the possible development of a resulting embolism or thrombus in the affected chamber which could lead to a myocardial infarction or stroke.
If given a choice, the patient suffering atrial fibrillation might well elect, nevertheless, to put off the shock until such time as he or she is resting, or is in a situation which is more convenient for acceptance of a shock applied to the heart. For example, the patient might be on a long drive when the atrial fibrillation occurred, at a time when a shock could cause a loss of control of the vehicle. The first problem then is for the patient to be aware of the atrial fibrillation, which, as pointed out above, is often unobserved except by an implanted device that would deliver an automatic shock; and the second problem is to give the patient an opportunity to possibly defer the defibrillating shock, at least briefly, to a more propitious time to receive treatment for the disorder.
Therefore, it is another aim of the invention to provide means for notifying the patient when an attack of atrial fibrillation occurs, before delivery of a shock as the automatic therapy delivered to treat the disorder, and to provide a way in which the patient can adjust the timing of the shock, within limits, according to the patient's convenience.